1. Field of the Invention
The present invention relates to an image processing apparatus which converts image data representing plural colors by a given number of bits into image data of a lesser number of bits, a control method which controls the image processing apparatus, a computer program which is used to achieve the control method, and a computer-readable storage medium which stores the computer program.
2. Related Background Art
There are various apparatuses which print color images, and the basis of these apparatuses is to print the image by using recording materials (ink, toner, etc.) of three colors, namely cyan (C), magenta (M) and yellow (Y), or four colors, namely C, M, Y and black (Bk).
Ordinarily, in a printer, the image data to be printed is converted into data for the recording materials of the colors provided in the printer in accordance with a predetermined color processing parameter, and the converted data is then actually printed. Moreover, in a printer of type of discharging ink droplets, an image is represented by binary data (i.e., data whose two values respective indicate ink discharging and no ink discharging), or represented by N values larger than binary by stepwise changing the number of ink discharges.
As quantization methods, a dither method, an error diffusion method and the like are known, and, moreover, various improved techniques are conventionally known in the error diffusion method.
For example, each of Japanese Patent Application Laid-Open Nos. 8-279920, 11-10918 and 2000-354172 discloses a method of achieving a uniformly distributed and visually desirable dot arrangement by quantizing two or three different colors as mutually correlating them.
Here, such a technique will be explained with reference to FIGS. 8A and 8B.
First, cyan and magenta are selected as the two different colors. FIG. 8A shows an example of the dot arrangement in a case where an image is extremely pale and thus print dots are few, and also in a case where ordinarily the error diffusion process is performed independently for each of cyan and magenta, as in a case of the first gradation value of 255 gradations.
As shown in FIG. 8A, since cyan dots and magenta dots do not at all correlate, these dots are mutually close together in certain areas or mutually separated in some cases, whereby these dots can form visually unpleasant patterns. Moreover, if a cyan dot and a magenta dot overlap each other, the result is a blue dot. Since the density of blue is high, blue dots are easily recognized by human eyes, whereby such blue dots degrade the image quality by causing an unpleasant graininess.
On the other hand, FIG. 8B shows an example of the dot arrangement in a case of quantizing the cyan and magenta dots while correlating their relative positions. In this method, since the cyan and magenta dots are arranged so that these dots mutually have appropriate distances, dark blue dots do not spuriously appear, nor does any improper dot-dense portion, and the result is visually preferable.
However, the above conventional techniques merely disclose the method of correlating the dots as a whole irrespective of the gradation values (densities) of respective colors. In other words, in the above conventional techniques, the light area where the dots are well seen and the dark area where the individual dots cannot be recognized because numerous dots are printed, are treated the same.
Moreover, the inventors of the present invention found that, if error diffusion is performed with correlation of mutually different colors in all gradations, a visually unpleasant pseudo-contour may appear, and tonality (or gradation capability) might deteriorate.
Even in ordinary error diffusion, there occurs a phenomenon in which, in a given portion of an image, errors do not accumulate and thus the appearance of dots is delayed relative to their location in the image data that is input to the error diffusion processing. This phenomenon is called a “sweep-together” phenomenon.
This phenomenon occurs in light-gradation areas, where the dots are very few, and in portions where a quantization output value begins to change in a case of multivalued recording. Conversely, there is also a phenomenon in which errors are numerous and thus dots appear excessively, which is called a “sweep-out” phenomenon. This phenomenon occurs, e.g., in the gradation from dark color to light color. In the method of mutually correlating different colors, there are many change portions of the quantization output values, whereby the “sweep-together” phenomenon and the “sweep-out” phenomenon tend to occur more frequently as compared with the ordinary error diffusion. Thus, pseudo-contours and dot-dense portions appear, whereby the image quality deteriorates.